Vascular diseases affect a large part of the world's population. Bypass surgery, whereby a conduit, either artificial or autologous, is grafted into an existing vessel to circumvent a diseased portion of the vessel or to restore blood flow around a blocked or damaged blood vessel, is one of the most common treatments for such diseases. It is estimated that over 1 million such procedures are performed annually.
Vascular grafts are also used as entry sites in dialysis patients. The graft connects an artery to a vein in the patient's body. A needle is inserted into the graft, blood is withdrawn, passed through a hemodialysis machine and returned to the patient through a second needle inserted in the graft.
Thirty to 50 percent of by-pass grafts fail within 5 to 7 years. The average life-span for hemodialysis grafts is even shorter, often less than two years.
A primary cause of graft failure is the closing of the graft due to tissue in-growth and eventually thrombosis formation. The smaller the diameter of a graft, the higher rate of failure. Numerous approaches to improving the performance of vascular grafts have been proposed. One such approach is the use of more biocompatible and durable synthetic materials in artificial grafts.
Among the synthetic materials that have been used in vascular grafts is polytetrafluoroethylene (PTFE, Teflon®), which has high durability complemented by good biocompatibility. However, PTFE and similar materials are still susceptible to thrombosis formation, which limits their utility. To counter this, the interior walls of PTFE grafts have been seeded with autologous endothelial cells (ECs) before implantation. Not only do ECs provide an excellent biocompatible surface, they also have substantial thrombolytic activity. In addition, ECs prevent neointimal proliferation and inflammatory reaction in the graft. However, EC-seeded grafts suffer from incomplete endothelialization and detachment of the endothelial cells from the surface of the graft due to the shear force of flowing blood.
To improve endothelialization, ECs have been genetically altered to express or over-express vascular endothelial growth factor (VEGF, U.S. Pat. No. 5,785,965). Not only can VEGF reduce the time from cell harvesting to seeding, it also permits use of lower initial graft seeding densities since rapid proliferation leads to faster graft coverage.
VEGF has advantages over other less EC-specific growth factors that can enhance endothelialization due to its reduced impact on other vascular cells, in particular smooth muscle cells (SMCs), and, as such, its reduced potential for causing adverse stimulatory effects. For instance, VEGF will recruit ECs, but not SMCs, from anastomosis sites.
While genetically altered ECs over-expressing VEGF resolve to a large extent the problem of incomplete endothelialization, detachment of cells from the interior surface of a graft under the shear stress of flowing blood still remains a problem.
One approach to dealing with the detachment problem has been to precoat the interior surface of a graft with an adhesive matrix to more solidly fix the cells to the surface. Also, exposing the cells seeded on the wall of the graft to continuous flow conditions during proliferation to simulate blood flow has been reported. While occurring at a slower rate, ECs still detach from the walls of grafts prepared using these techniques and thus the useful life span of the grafts remains sub-optimal. In addition, the detached cells leave extracellular matrix, which is highly thrombogenic, on the grafts after detachment.
Thus, there remains a need for endothelialized vascular grafts in which the ECs can withstand the shear force of flowing blood for a longer time. This invention relates to cells that have been genetically altered to endow them with properties that enable them to resist the shear force of flowing blood and thereby to give artificial vascular grafts having greater patency times.